During a start-up phase, in which the burner is ignited and which may last for about a minute, such a catalytic burner must be supplied with gas at a higher rate than is necessary for sustaining a flame during a subsequent operating phase. It is also desirable to provide means for varying the flow rate of the gas during that operating phase, either manually or automatically in response to changes in ambient temperature, as well as means for continuously monitoring the atmosphere of the burner in order to detect a possible interruption of the gas flow.
Thus, a mechanism designed to control the supply of heating gas to the burner should be able to perform the three functions of (a) temporarily increasing the flow rate on start-up, (b) varying the reduced flow rate in the subsequent operating phase and (c) shutting off the flow in the event of an extinction of the flame. Ordinary valves of the rotary-cock type are not suitable for this purpose since they generally can establish only a limited number of working positions corresponding to maximum flow rate, reduced flow rate and cutoff. Slide valves, such as those of the needle type, are more versatile but are also difficult to adapt to the aforestated requirements.
If, as is usually the case with mobile units, the burner is not equipped with a permanently lit pilot flame, an igniter must be activated during the start-up phase. The control mechanism for such a unit, therefore, will also have to be designed to operate the igniter.